1. Field of the Invention
The disclosure generally relates to superconductor circuits and more particularly to superconductor circuits using Rapid Single Flux Quantum (RSFQ) logic and a method and apparatus for controlling the same.
2. Description of Related Art
A classical computer operates by processing binary bits of information that change state according to the laws of classical physics. These information bits can be modified by using simple logic gates such as AND and OR gates. The binary bits are physically created by a high or a low energy level occurring at the output of the logic gate to represent either a logical one (e.g. high voltage) or a logical zero (e.g. low voltage). A classical algorithm, such as one that multiplies two integers, can be decomposed into a long string of these simple logic gates. A set of such gates is said to be complete if all possible algorithms can be generated from only that set of gates. For example, the classical NAND gate by itself forms a complete set.
Like a classical computer, a quantum computer also has bits and gates. But instead of using logical ones and zeroes, a quantum bit (“qubit”) uses quantum mechanics to occupy both possibilities simultaneously. This ability means that a quantum computer can solve a large class of problems with exponentially greater efficiency than that of a classical computer.
It is widely known that a combination of single-qubit operations with a two-qubit controlled-not (CNOT) gate forms a complete set for quantum computation. It has been demonstrated that some single qubit operations can be performed by coupling the qubit to a resonator. An objective of ongoing research in this field is to develop a more efficient means of achieving arbitrary qubit operations.
Devices based upon the characteristics of a Josephson Junction are valuable in high speed circuits. Josephson junctions can be designed to switch in times of a few picoseconds. Their low power dissipation makes them useful in high-density computer circuits where resistive heating limits the applicability of conventional switches. Parallel Josephson junctions are used as active elements in superconducting quantum interference devices (“SQUIDs”) for the detection of minute magnetic fields. A conventional SQUID comprises two Josephson Junction elements coupled by an inductor. The SQUID stores a flux quantum and the magnetic field of the SQUID is quantized to a value proportional to the Planck's constant.
Rapid Single Flux Quantum (RSFQ) logic can provide high speed, low power control of superconductive qubits based on Josephson Junctions. RSFQ is a highly developed family that operates at clock speeds of 100 GHz. It has unique analog properties that make its control signal accurate and repeatable due to the quantization of the magnetic flux in a superconductive circuit loop. When coupled together, RSFQ circuits can transfer flux quanta between each other. The presence or absence of flux quanta determines the state of the circuit as 0 or 1.
Conventionally, the Josephson Junction is supplied with a DC bias and the power budget in such circuits is dominated by static power consumption which happens whether or not the active device is switching. It is important to reduce power consumption including elimination of such static power dissipation in such circuits. It is also important to devise proper means for controlling the operation of such circuits.
In RSFQ logic, information is stored in superconductor loops as tiny magnetic flux quanta and a bit is transferred as several picosecond-wide voltage spike with a quantized area of approximately 2.07 mV ps. The tiny and quantized nature of magnetic flux quanta significantly (by several orders of magnitude) reduces crosstalk and power consumption as compared to CMOS devices. The RSFQ circuit can be considered as having elementary cells or timed gates. Each cell has two or more stable flux states. The cell is fed by SFQ input pulses S1, S2, . . . Si that can arrive from one or more signal lines and a clock timing line T. Each clock pulse marks a boundary between two adjacent clock periods by setting the cell into its initial state. During the new period, an SFQ pulse can arrive or not arrive at each of the cell inputs Si. Arrival of the SFQ pulse at a terminal Si during the current clock period defines the logic value 1 of the signal Si while the absence of the pulse during this period defines the logic value 0 of this signal.
RSFQ circuits do not require the exact coincidence of SFQ pulses in time nor is a specified time sequence of the various input signals needed. Each input pulse can either change or not change the internal state of the cell. Input pulses cannot produce an immediate reaction at the output terminal(s) Sout. Only the clock pulse T is able to fire out the pulse(s) Sout corresponding to the internal state of the cell predetermined by the input signal pulses that have arrived during the clock period. The same clock pulse terminates the clock period by resetting the cell into its initial state. An elementary cell of the RSFQ family is approximately equivalent to a typical asynchronous logic circuit coupled with a latch (flip-flop) that stores its output bit(s) until the end of the clock period. There is a need for a method and apparatus for controlling the logical state of the quantum bit.